1. Field of the Invention
The present invention relates to a process for producing a multilayer ceramic circuit board with copper, which comprises a glass-ceramic insulator having a low dielectric constant and a conductor having a low electrical resistance.
2. Description of the Related Art
Multilayer ceramic circuit boards have been produced by using either a high melting point metal such as molybdenum or tungsten, or a noble metal such as gold. Recently, it is desired to use copper as a conductor, because its electric resistance is lower than the former and its price is cheaper than the latter. The lower melting point of copper enables the use of a glass-ceramics having a softening point far lower than alumina. It is, of course, necessary to fire the glass-ceramics in an inert atmosphere in order to prevent oxidizing of the copper. On the other hand, it is desirable that the organic materials are completely burnt out without leaving residual carbon, which may derive from a binder resin, a plasticizer, a deflocculant, and a solvent. These organic materials bring the following improvements in the behavior of a ceramic slurry and green sheet:
(a) The solvent dissolves the other organic materials to distribute them uniformly in the slurry, to disperse the ceramic paricles in the slurry, and the solvent evaporates from the green sheet formed by doctoring the slurry and leaves micropores in the green sheet.
(b) The deflocculant forms a thin coating film on the surface of the ceramic particles, to distribute them uniformly in the formed slurry and stabilize their distribution due to the occurrence of steric hindrance and electric repulsion between the ceramic particles. Thus, it is possible to form a green sheet in which the ceramic particles are uniformly distributed without flocculation.
(c) The binder dissolves in the solvent and enhances its viscosity to form a thixotropic slurry. Thus, it is possible for the ceramic particles to bind with each other in a green sheet.
(d) The plasticizer structurally expands the binder and improves distribution of the binder in the slurry. Thus, it is possible to obtain a flexible green sheet.
Although these organic materials exhibit such desirable effects in forming a ceramic slurry and green sheet, one of them, i.e., a deflocculant, is sometimes eliminated in order to decrease the total amount of organic materials, thereby decreasing the residual carbon after firing.
Nakamura et Kamehara to Fujitsu Ltd., disclose in Japanese Examined Patent Publication No. 52-5523 to produce a ceramic board by firing a green sheet prepared from a ceramic slurry which comprises alumina particles, a deflocculant, i.e., sorbitan sesquioleate or sorbitan trioleate, a hydrophobic binder, and a plasticizer, first in normal atmosphere at 400.degree. C. to 600.degree. C. to eliminate the binder and then at 1300.degree. C., and finally in a hydrogen atmosphere at 1600.degree. C.
Honda to Fujitsu Ltd., discloses in Japanese Unexamined Patent Publication No. 51-127112 a process for producing a multilayer glass circuit board by laminating conductor paste-printed glass green sheets prepared from a glass slurry which comprises glass powder, a binder, e.g., polymethacrylate or polyvinylbutyral resin, a plasticizer, e.g., dioctyl phthalate, dibutyl phthalate, a deflocculant, e.g., nonyl phenol ethylene oxide adduct (registered trade name "Tergitol NPX" available from Union Carbide Ltd.), a solvent, e.g., methyl ethyl ketone and other adduct, e.g., polyethylene glycol; prefiring the laminated structure at about 500.degree. C. in a reducing atmosphere, and then firing the structure in vacuum.
Tormey et al reports in MIT Industrial Liaison Program Report 6-15-84, 1984 to use a fish oil or glycerol trioleate as a deflocculant in a ceramic slurry.
Herron et al to IBM Ltd., disclose in U.S. Pat. No. 4,234,367 to produce a ceramic circuit board in an atmosphere of water vapor containing hydrogen in a ratio of H.sub.2 :H.sub.2 O 10.sup.-4 to 10.sup.-6.5 by firing a ceramic structure, which comprises copper as a conductor, spodumene or cordierite as a ceramic insulator, polyvinylbutyral as a binder, and dioctyl phthalate or dibutyl phthalate as a plasticizer. However, there is no reference in the description to a deflocculant. In addition, polyvinyl butyral is not thermally depolymerizable and does not completely burn out at a temperature lower than 1000.degree. C. in an non-oxidizing atmosphere. Thus, the inventors consider that of the above fired board will inevitably deteriorate the mechanical and electrical properties due to its porous structure and to residual carbon.
Kamehara et al to Fujitsu Ltd., disclose in U.S. Pat. No. 4,504,339 a method for producing a multilayer glass-ceramic structure having copper-based conductors therein for use as a circuit board. In this method, a multilayer structure consists of layers of a thermally depolymerizable resin and glass-ceramic containing preferably 40 percent to 60 percent by weight of Al.sub.2 O.sub.3 and layers of a copper-based paste. The structure is fired in an inert atmosphere containing water vapor, the partial pressure of which is 0.005 to 0.3 atmosphere, preferably at 550.degree. C. to 650.degree. C. The structure is then sintered in a nitrogen atmosphere containing no water vapor, preferably at about 900.degree. C.
However, if the firing temperature in the inert atmosphere containing water vapor is higher than 650.degree. C., the residual carbon is trapped in the closed pores in which H.sub.2 O vapor is present. The carbon then reacts with the H.sub.2 O to form CO.sub.2. This phenomenon results in bloating of the glass-ceramic. Thus, Kamehara et al teach the composition of a firing atmosphere and usage of a thermally depolymerizable resin, e.g., polymethyl methacrylate, but do not describe the use of a deflocculant in the organic materials, and a silica rich SiO.sub.2 -B.sub.2 O.sub.3 type glass in the ceramics.
A slurry without a deflocculant is generally poor in the dispersion property of ceramic particles in the solvent containing a binder and a plasticizer, and green sheets formed from such a slurry exhibit the same demerit as the slurry. In addition, a fired multilayer structure formed by laminating such green sheets exhibits deviation of the shrinkage rate and the mechanical strength in one and the same structure.